Toxoplasma gondii is an important protozoan parasite of man and animals. In recent years, its importance as a human pathogen has grown with its appearance as a potentially fatal opportunistic pathogen in AIDS patients. Our primary objective is to determine the structure and function of several major antigens from the growing forms of this parasite and use this information to control the disease. We are also interested in the biological functions of some of these molecules from the perspective of the parasite. To date, we have identified several recombinants which appear to encode the major surface antigen, a penetration-enhancing factor and several other antigens of unknown function. All the recombinants are being characterized by nucleotide sequence analysis and with respect to their in vivo coding function. in this application we request funds to continue this work with the ultimate goal of assessing the utility of these recombinant antigens to serve as the source of material for immunization and/or sero-diagnosis. They will be further used in studies on the basic biology of the parasite with the aim of determining the structure/function relationship for each molecule.